1. Field of the Invention
The present invention relates to a polymer electrolyte and a fuel cell using the same, and more particularly, to a polymer electrolyte used to form a polymer electrolyte membrane that has excellent ionic conductivity at high temperatures and low humidity and reduces methanol crossover.
2. Discussion of the Background
Fuel cells are electrochemical devices that convert the chemical energy of hydrogen and oxygen into electricity. The hydrogen may be obtained from hydrocarbons, such as methanol, ethanol, and natural gas. The energy converting process used by fuel cells is substantially effective and environmentally friendly, and various types of fuel cells have been developed.
Fuel cells can be categorized according to the type of electrolyte used. Types of fuel cells include phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), polymer electrolyte membrane fuel cells (PEMFCs), alkali fuel cells (AFCs), and the like. These fuel cells operate based on the same principle, but fuels, operating temperatures, catalysts, electrolytes, and the like may vary according to the type of fuel cell.
PEMFCs are particularly advantageous for use in small stationary electric generating equipment and transportation systems because they can operate at low temperatures, have a large output density, can be started quickly, and can quickly respond to changes in the required power output.
One important component of PEMFCs is a membrane electrode assembly (MEA). An MEA may includes a polymer electrolyte membrane and two electrodes respectively acting as a cathode and an anode, which are attached to both sides of the polymer electrolyte membrane.
The polymer electrolyte membrane may act as a separator to prevent direct contact between an oxidant and a reductant, an insulator to electrically insulate the two electrodes, and a proton conductor. To serve these purposes, the polymer electrolyte membrane properties may include high proton conductivity, excellent electrical insulating property, low permeability to reactants, excellent thermal, chemical, and mechanical stability under the fuel cell operating conditions, low manufacturing costs, and the like.
Various polymer electrolyte membranes have been developed in attempts to produce a polymer electrolyte membrane with the desired properties. Perfluoropolysulfonic acid films, such as films made of NAFION by DuPont, are widely used due to their durability and performance. However, perfluoropolysulfonic acid films must be sufficiently humidified for proper operation and must be used at 80° C. or lower to prevent the loss of humidity. Additionally, perfluoropolysulfonic acid films are unstable under fuel cell operating conditions because C—C bonds in its molecular backbone may be attacked by O2.
Furthermore, in DMFCs, an aqueous methanol solution is supplied to the anode as fuel. A portion of the unreacted methanol may permeate into the polymer electrolyte membrane. The methanol permeating into the polymer electrolyte membrane may diffuse through and swell the polymer electrolyte membrane and then enter a cathode catalyst layer. This phenomenon is referred to as methanol crossover. The methanol is directly oxidized in the cathode where electrochemical reduction between hydrogen ions and oxygen occurs, thereby lowering a potential in the cathode and deteriorating the performance of the cell.
PEMFCs that can operate at temperatures of 100° C. or higher are desirable because the efficiency of a catalyst and the resistance of the catalyst against CO poisoning of the catalyst increases. In addition, a fuel cell that can operate at high temperatures and low humidity may not require peripheral devices to humidify the fuel cells. This may simplify the overall structures of the fuel cell systems.
Attempts at producing a PEMFCs that may be operated at high temperatures and low humidity have included a polymer membrane produced by fixing a solid, such as silicagel, zirconium phosphoric acid, or the like, therein. However, the improvement in performance of this type of polymer membrane is limited. A polymer membrane impregnated with an acid has been developed, but the polymer membrane suffers from a loss of the acid and corrosion over time. A membrane using an ionically non-conductive polymer in which an acid is distributed has been developed. However, in this membrane, a loss of acid and corrosion occur, and anions are adsorbed onto a catalyst. A method using CsHSO4 as a solid acid or a solid acid salt has been tried, but the manufacturing process for this membrane is complex, and the mechanical properties of the membrane are unsatisfactory due to high solubility in water.
A need thus exists for a polymer electrolyte membrane that has satisfactory ionic conductivity and durability at high temperatures and low humidity.